ii2 PHYSIOLOGICAL PHYSICS. [Chap. xi. 



ELECTRODES. He took advantage of a discovery of 

 Regnanld, that a strip of chemically pure zinc plunged 

 into a solution of neutral sulphate of zinc exhi- 

 bited no polarisation, a discovery to which Matteucci 

 added the fact that ordinary zinc, properly amal- 

 gamated, had the same property when plunged in a 

 saturated solution of the sulphate. Du Bois, there- 

 fore, constructed troughs, of the shape shown in Fig. 

 58, made of zinc, and insulated by having a base of 

 vulcanite. The inner surface of the trough is care- 

 fully amalgamated, and the outer surface coated with 

 a layer of black varnish, to prevent the sulphate getting 

 access to any unamalgamated zinc. By an insulated 

 handle g the trough can be lifted, while on the base is 



a binding screw k, for the at- 

 tachment of wires. Cushions, 

 called DERIVING CUSHIONS, 

 are made of white Swedish 

 filtering paper. They must 

 be thick enough to fill up 

 the cavity of the troughs. 

 The sides should be perpen- 

 dicularly cut with a sharp 

 razor. The various layers of 

 the cushion should be secured 



Fig. 58.-Non-Polarisable Elec- by stitching at one end. The 



trodes. J .. . c , , . , 



cushion is placed m tne 



trough, and folded over the lip, the projecting part 

 being terminated by a perpendicular section 6. The 

 cushion is soaked in the zinc solution before being 

 finally arranged in the trough, and, when placed, is 

 retained in position by a plate of ebonite s, and an 

 indiarubber band; the trough is then filled up 

 with the saturated solution of zinc. Two such troughs 

 are prepared and put into connection with the wires 

 from the galvanometer. It is easily seen that if both 

 are "not supplied with the same strength of zinc 



